The sensitive and accurate detection or identification of individual molecules from biological and other samples has proven to be an elusive goal, with widespread potential uses in medical diagnostics, pathology, toxicology, environmental sampling, chemical analysis, forensics and numerous other fields. Attempts have been made to use Raman spectroscopy or surface plasmon resonance to achieve this goal. When light passes through a medium of interest, a certain amount of the light becomes diverted from its original direction in a phenomenon known as scattering. Some of the scattered light also differs in frequency from the original excitatory light, due to the absorption of light and excitation of electrons to a higher energy state, followed by light emission at a different wavelength. The difference of the energy of the absorbed light and the energy of the emitted light matches the vibrational energy of the medium. This phenomenon is known as Raman scattering, and the method to characterize and analyze the medium or molecule of interest with the Raman scattered light is called Raman spectroscopy. The wavelengths of the Raman emission spectrum are characteristic of the chemical composition and structure of the Raman scattering molecules in a sample, while the intensity of Raman scattered light is dependent on the concentration of molecules in the sample.
The probability of Raman interaction occurring between an excitatory light beam and an individual molecule in a sample is very low, resulting in a low sensitivity and limited applicability of Raman analysis. It has been observed that molecules near roughened silver surfaces show enhanced Raman scattering of as much as two orders of magnitude or more. This surface enhanced Raman scattering (SERS) effect is related to the phenomenon of plasmon resonance, wherein metal nanoparticles or metal coatings exhibit a pronounced optical resonance in response to incident electromagnetic radiation, due to the collective coupling of conduction electrons in the metal. In essence, nanoparticles of gold, silver, copper and certain other metals can function to enhance the localized effects of electromagnetic radiation. Molecules located in the vicinity of such particles exhibit a much greater sensitivity for Raman spectroscopic analysis. Surface enhanced Raman spectroscopy (SERS) is the technique to utilize surface enhanced Raman scattering effect to characterize and analyze the medium or molecule of interest.
Attempts have been made to exploit SERS for molecular detection and analysis, typically by utilizing metal nanoparticles or fabricating rough metal films on the surface of a substrate and then applying a sample to the metal nanoparticles in liquid or the metal-coated surface. However, the metal particles can aggregate to yield stronger resonance, and the enhancement factor with the metal particles is, in general, higher than that with the metal-coated surface. To date, sodium chloride has been identified as a chemical that overall enhances the SERS signal when applied to the metal nanoparticles or metal-coated surfaces before or after the molecule of interest is introduced. However, using sodium chloride as an enhancer has not been sensitive enough to detect lower concentrations of target molecules such as single nucleotides reliably, and as a result SERS has not been suitable for DNA sequencing. Thus, there lies a need to reliably detect individual molecules such as nucleotides using a SERS process.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.